In general, an alarm device of a car is a triangular plate which is stand on a road to alert other drivers not to near a destroyed car. In some designs, the triangular plate is adhered with a light reflecting paper. However, it's effect is insufficient at night since the reflecting light is too weak to alert the user. In some design, some complicated structure is developed as an alarm plate for alert the driver, but the structure is complicated and must be assembled with a complex operation with more time being necessary. Moreover, to store the alarm plate is also another trouble to the user.